Polycarbonate resins are thermoplastic resins having excellent impact resistance and heat resistance and widely used as parts in the fields of machinery, automobiles, electricity and electronics. In particular aromatic polycarbonate resins have a high glass transition temperature and are expected to exhibit high heat stability. However, they frequently fail to show sufficient flowability in processing. Therefore, aromatic polycarbonate resins should be processed at relatively high processing temperatures around 300.degree. C. In molding aromatic polycarbonate resins by, for example, injection molding, relatively high injection speed and pressure are required.
On the other hand, thermoplastic polyester resins are excellent in mechanical properties, electrical properties, and chemical resistance and exhibit satisfactory flowability on being heated at or above their crystal melting point and therefore have been used widely as fiber, film and a molding material.
It has been attempted to improve the flowability and the like of polycarbonate resins by taking advantage of these characteristics of the thermoplastic polyester resin. For example, JP-B-36-14035 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-B-39-20434, and JP-A-59-176345 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") propose a polycarbonate resin composition containing a polyester resin, such as polyethylene terephthalate, polybutylene terephthalate, etc.
In order to secure safety against fire, thermoplastic resins are often required to have such flame retardance as to meet the standards of UL-94 V-0 or 5V (Underwriter's Laboratories Standard, U.S.A.). Various flame retardants have been developed and studied for this purpose.
Recent environmental concerns growing particularly in Europe have promoted the study on the use of halogen-free flame retardants, such as phosphorus type flame retardants. Useful phosphorus type flame retardants include organic phosphorous compounds and red phosphorus.
Known organic phosphorus compounds include those disclosed in JP-A-63-227632, JP-A-5-1079, and JP-A-5-279513. Compositions which are made flame-retardant by addition of an organic phosphorus flame retardant include the flame-retardant resin composition of JP-A-5-179123, which comprises a polycarbonate resin and other resins and contains an organic phosphorus flame retardant, a boron compound, organopolysiloxane, and a fluororesin, and the flame-retardant resin composition of JP-A-6-192553, which comprises a polycarbonate resin and a polyalkylene terephthalate resin and contains a graft copolymer, an oligomeric organic phosphorus flame retardant, and a fluorinated polyolefin.
Known red phosphorous species include those described in JP-B-54-39200, JP-A-55-10463, and JP-B-5-8125. Compositions which are made flame-retardant with red phosphorus include flame-retardant resin compositions comprising a polycarbonate resin and powdered red phosphorus as disclosed in JP-A-48-85642 and JP-A-50-78651.
Red phosphorus is difficult to handle because for one thing it is a dangerous chemical having a danger of dust explosion and for another it tends to emit smell or gas when processed in high temperature. In order to overcome these problems, various techniques for coating the surface of red phosphorus for stabilization have been proposed. For example, JP-A-52-142751, JP-B-5-18356, and JP-A-5-239260 disclose red phosphorus coated with a thermosetting resin, aluminum hydroxide, etc. or electrolessly plated red phosphorus and thermoplastic resins which are rendered flame-retardant by addition of the thus stabilized red phosphorus.
JP-B-2-37370 proposes a flame-retardant resin composition comprising a polyester resin and thermosetting resin-coated red phosphorus and, if desired, a reinforcing filler. JP-A-5-239260 and JP-A-5-247264 disclose a flame-retardant resin composition comprising a thermoplastic resin such as a polycarbonate alloy, a polyester resin, etc. and electrolessly plated red phosphorus.
In the fields where such flame-retardant resin compositions are used as, for example, electric and electronic parts, simplification of assembly and cost reduction have been desired, and it has been promoted to make parts integral or thinner. Therefore, materials used in these parts are required to show satisfactory flowability in molding and to maintain high heat resistance and high flame retardance.
However, addition of an organic phosphorus flame retardant to a polycarbonate resin in an attempt to impart sufficient flame retardance results in considerable reduction in heat resistance.
Polycarbonate resin compositions containing red phosphorus or stabilized red phosphorus lack long-term heat stability. That is, moldings obtained suffer from deformation when exposed to a temperature no higher than around 150.degree. C. for a long time. Besides, the compositions have poor molding processability because of low flowability. If the compositions are molded at high temperatures to secure flowability, there arise different problems such that a smell attributable to red phosphorus issues during molding and that decomposition gas generates during molding to contaminate the mold.
In addition it is difficult with red phosphorus alone to obtain sufficient flame retardance. It means that red phosphorus should be used either in a large quantity or in combination with another flame retardant or a flame retardation aid. However, addition of a large quantity of red phosphorus leads to a stronger smell attributable to red phosphorus, and a combined use of a flame retardation aid results in not only destruction of the balance of properties of the resin but an increase of cost.